I think some people are making the assumption that where Nicola was found is where she's been for the last 3 weeks and that police have somehow missed her. I don't think that's the case. People may not appreciate how powerful a tidal river can be. It's entirely understandable...we all think of rivers only moving in one direction, from source to sea, but that's not the case in a tidal river such as this stretch of the Wyre..
This video shows a tidal bore on the Wyre from 2015 and the last section of the video is filmed at Cartford Bridge which is a mile or so downstream from where Nicola was discovered.
The river is tidal from the sea to the weir, just downstream from where Nicola went missing. When you watch it, remember that what you're seeing is the tide coming in and pushing water up the river.
The water is running from the sea towards the weir, in effect turning the direction of flow of the river (bit more complicated than that, but essentially the case).
Look at the power that's visible at the surface and imagine the turbulence below.
Also bear in mind that what you're seeing is just the start of the tide coming into the river. There's a lot more water that follows in the hours after as the high tide peaks. We also know that Nicola was discovered just after a particularly high tide. Remember that there are 2 high tides per day.
It's very sad to say, but Nicola may have been being pushed up and down the river. Or she may have been snagged somewhere further downstream and the high tide has released her and pushed her back up the river to where she was found after the tide receded.